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Malware authors have ported a Trojan originally written for Linux systems to hijack Mac OS X systems, security researchers found. Once compromised, the Macs could be used to launch denial-of-service attacks.

The Tsunami Trojan works by latching onto a host. Tsunami appears to be derived from Kaiten, an old backdoor Trojan, dating back at least to 2002, that was designed to infect Linux systems, Robert Lipovsky, a malware researcher at ESET wrote in an Oct. 25 blog. The Trojan is also evolving pretty quickly, as ESET researchers discovered at least one new variant by Oct. 27, according to Pierre-Marc Bureau, a senior malware researcher at ESET.

The Trojan is named for its likely goal, to force infected computers into becoming part of a compromised network to launch distributed-denial-of-service (DDoS) attacks, flooding Websites with traffic and causing them to stop responding, according to Graham Cluley, a senior technology consultant at Sophos.

"Even though there is far less malware in existence for Mac OS X than for Windows, that doesn't mean the problem is nonexistent," Cluley wrote on the Naked Security blog.

Further reading

Once the Trojan program has been copied onto the system, either intentionally or maliciously, the malware attempts to connect to an Internet Relay Chat channel to receive further instructions. The malware can launch DDoS attacks at a targeted server, download additional malware on the machine and provide remote access to the system, Cluley said.

The new variant has the ability to make itself launch automatically on reboot, causing it to be more persistent on the system, Bureau said. It also had an updated command and control IRC server and channel than what was included in the previous version.

There are "very few hosts" infected with this malware, Bureau said, noting that the developers are most likely still in the process of testing the application.

The Kaiten source code has been publicly available since at least September 2009, and it was "trivial" to compile the code, using Apple's XCode, to create a Mac-executable, Mac security company, Intego noted on its blog. Intego also noted that people may intentionally install the Trojan on their systems if they voluntarily take part in DDoS attacks, such as those supporting Anonymous operations.

Those "volunteers" have "effectively put control" of their Macs into someone else's hands, Cluley said.

While there have been many instances of Windows malware re-engineered for the Mac platform, Tsunami appears to be the first that takes advantage of the fact that Mac OS X is based on BSD, an operating system with many similarities with Linux.

"If the bad guys think they can make money out of infecting and compromising Macs, they will keep trying," Cluley said, predicting more malware targeting "poorly defended Mac computers."

There has been a flurry of Mac malware activity recently. Beginning in September, F-Secure researchers found the Revir/Imuler Trojan, which was spread through malicious PDF files. Another Trojan-called Flashback because it masquerades itself as an update to Adobe Flash, or Flash Player installer-was detected in late September and went through various iterations this month, according to F-Secure. New capabilities include the ability to detect if the Mac had a firewall installed as well as the ability to search for virtual machines. If these were found, the malware deleted itself.

The latest Flashback was far more sinister, as it could disable Apple's built-in Xprotect malware detection system by overwriting certain Xprotect files to prevent the system from getting new signature updates. Intego said Flashback was the first Mac malware that could "intentionally" damage system files. Removing the malware and restoring the system could be time-consuming, Intego noted.

"We hope Mac malware doesn't use similar techniques in the future that would require a full installation of Mac OS X to repair damage," Intego said.